A man named Tsugami Shouichi has no idea who he is; no memories of anything regarding anything prior to that fateful day he washed up on a beach with a letter addressed to a "Tsugami Shouichi". Coincidentally, mysterious creatures named Unknown are committing murders and a policeman named Hikawa Makoto has become Kamen Rider G3 in order to fight them. Coincidentally yet again, a swimmer named Ashihara Ryou has been affected too. How will these three fates go? How will these fates intervene?

There's some references to Kuuga in Agito. However, it is unlikely that they are canon because of varying conflicting facts. Toei also has said that they want Agito to be treated as a stand alone series.